The production of ethylene-vinyl acetate (EVA) copolymers is well known in the art, and there is a continuing effort to find improvements in these copolymers, especially by terpolymerization of other terpolymerizable monomers. Stiles in U.S. Pat. No. 3,965,062 discloses, for example, that EVAI terpolymers exhibit increased melt indices in combination with lower stiffness and Vicat softening temperatures and are superior hot melt coating resins.
The halogenation of olefin polymers and copolymers is also well known in the art. Chlorinated polyethylenes, for example, are commercial products. Many of these products are elastomeric in nature since the residual crystallinity of polyethylene is gradually lost as chlorine is introduced into the polymer by the essentially random nature of the chlorination process. Accordingly, the relative molecular weight of chlorinated polyethylene elastomers is often judged from the Mooney viscosity, a measurement commonly employed in the rubber industry.
In U.S. Pat. No. 4,277,579, Beck, et al. show that the molecular weight of chlorinated ethylene polymers, including EVA copolymers, is increased by treatment with a small amount of a sulfur compound (S.sub.2 Cl.sub.2, SCl.sub.2, etc.) either before or during chlorination. As the degree of chlorination increases, Beck, et al. found that the Mooney viscosity of the chlorinated elastomeric products increased. Similar results were reported by Humbert, et al. [Die Makromolekulare Chemie, 175, 1597 (1974)] who determined that the degree of polymerization, i.e., polymer chain length, passes through a maximum and then decreases as the degree of chlorination of polyethylene increases, irrespective of whether the chlorination is done in solution or in dispersion. No sulfur compound was added in the chlorination processes of Humbert, et al. Finally, as will be disclosed hereinafter, in preparing certain chlorinated EVA elastomers with different degrees of chlorination for comparative purposes, we observed that Mooney viscosity rises significantly with increasing chlorine content of the chlorinated EVA elastomers, also without the addition of a sulfur compound.
Accordingly, it is an object of this invention to provide a halogenated and especially a chlorinated EVAI terpolymer.
It is another object of this invention to provide a halogenated and especially a chlorinated EVAI terpolymer that has superior heat stability, without added heat stabilizer, compared to that of chlorinated EVA copolymers of similar vinyl acetate content.
It is a further object of this invention to provide a halogenated and especially a chlorinated EVAI terpolymer exhibiting superior heat stability, without added heat stabilizer, at 200.degree. C. as measured by the time required for the appearance of first yellow color.
These and other objects have been achieved according to the present invention.
Although Beck, et al. in U.S. Pat. No. 4,277,579 mention the chlorination of ethylene-butene copolymers, there is no disclosure therein of chlorinated EVAI terpolymers. In the present invention, a halogenated and especially a chlorinated EVAI terpolymer is provided which unexpectedly exhibits improved thermal resistance as compared to chlorinated EVA copolymers of similar vinyl acetate content prepared by the same synthesis procedures.
In the present invention, a halogenated and especially chlorinated EVAI terpolymer having improved thermal stability is provided wherein the amount of halogen does not exceed above about 25 weight percent based on the terpolymer. At halogen levels above about 25 weight percent, even with isobutylene present in the polymer, the improved thermal stability properties are not achieved.